Scale Inhibitor

ABSTRACT

This invention relates to a process for inhibiting scaling in a subterranean oil or gas producing formation, comprising adding to the formation a composition comprising a metal chelant, a scale inhibitor and divalent metal cations, wherein the stability constant of the metal chelant—metal cation chelate at ambient temperature is equal to or higher than the stability constant of the chelate formed from the metal cations and the scale inhibitor, and wherein the solubility of the chelate formed from the metal cations and the scale inhibitor decreases with increasing temperature.

This invention relates to a novel scale inhibitor composition, and to aprocess of inhibiting scale during oilfield operations by applying saidscale inhibitor to the water or water/oil emulsion obtained duringoilfield operations.

BACKGROUND

Water from natural sources often contains dissolved minerals, with anappreciable presence of ions such as Ca²⁺, Mg²⁺ and, in the case ofoilfield formation water, Ba²⁺, Sr²⁺ and Ra²⁺. Under conditions oftemperature or pH change, loss of carbon dioxide from solution oradmixture with other water containing different mineral content,relatively insoluble species such as carbonates and sulphates maydeposit from solution as scale. In offshore oilfields such depositionmay be particularly acute when sulphate-containing seawater, pumpedunderground to aid oil recovery, comes into contact with formationwater.

Deposited scale impedes oil recovery and may even become severe enoughto block an oilwell. It is therefore a common procedure to treatoilwells with a scale inhibitor to minimize or prevent scale deposition.

In use, a relatively concentrated solution of the scale inhibitor ispumped down the oilwell and flushed out into the formation. From here itleaches back into the produced water, protecting the well and thepipework from scaling.

A careful balance of properties must be achieved. The scale inhibitordoes not only have to control scale, but must also on the one hand havesufficient solubility in the waters at the temperatures it will meet toenable placement in the formation without itself prematurelyprecipitating from solution, whilst on the other hand it must retainstrongly enough with in the formation rock to give a suitable slow leachrate. If the scale inhibitor does not adsorb strongly enough it will allleach back very quickly and the well will require re-treatment after ashort time. The retention of scale inhibitor can be achieved byadsorption and precipitation processes.

Precipitation is a known method for achieving longer scale inhibitorsqueeze lifetimes.

PRIOR ART

U.S. Pat. No. 3,633,672 discloses a method of injecting into a formationa scale inhibitor which forms only slightly soluble salts of theinhibitor in neutral to alkali pH. This is performed by injecting acomposition containing a highly acidic solution, the inhibitor and asoluble multivalent salt. When the acid is neutralized by the formationthe pH rises and the slightly soluble inhibitor salt precipitates. Thisthen slowly dissolves in the produced fluids protecting the productionequipment. In practice this technology has been found to be unreliableas there is little control over the rate of reaction and the placementof the precipitate. The formation must be basic in nature hence it isnot applicable to a wide range of wells.

U.S. Pat. No. 4,393,938 discloses a method for injecting an acidicsolution containing a mixture of scale inhibitor, multivalent andmonovalent cations. The multivalent cationic salt of the scale inhibitoris then formed by ion exchange within the formation due to bufferingaffect of the formation. This method is limited by reservoir type andmineralogy and is often ineffective due to poor yield of theprecipitation reaction.

U.S. Pat. No. 5,141,655 discloses a method for injecting an acidic scaleinhibitor composition including a soluble multivalent salt and a ureatype compound. On placement downhole the urea type compound is thermallydecomposed to form a basic solution. At the higher pH the cationic saltof the scale inhibitor is formed and precipitates. At lower temperaturese.g. 40-60° C. the urea type compound would not decompose, the pH wouldremain the same and the scale inhibitor salt would not precipitate. Thisprocess is very much temperature driven and is limited to reservoirswith a bottom hole temperature greater than 80° C.

U.S. Pat. No. 4,860,829 and SPE17008 disclose a method for scaleinhibitor injection utilizing a metal chelant with a stability constantlower than the phosphonate scale inhibitor, and a dissolved multivalentsalt. This results in the release of the multivalent cations from thechelant. The phosphonate inhibitor then precipitates as the cationicsalt. The precipitation reaction occurs immediately on mixing with theother components. Premature precipitation can cause poor productplacement in the reservoir. It is claimed that a delay in precipitationcan be caused by altering pH. This method is limited as the chelatingagent must have a lower stability constant than the phosphonate, thusonly a narrow selection of scale inhibitors can be used. Temperature isalso a limiting factor as at higher well temperatures the scaleinhibitor will precipitate too quickly and product placement will not beaccurate.

The scale inhibitors of the prior art, particularly the compositionaccording to U.S. Pat. No. 4,860,829 have the disadvantage of lowstability when mixed. Therefore, it is required that the scale inhibitorcomponents according to U.S. Pat. No. 4,860,829 have to be storedseparately before their deployment. It has been the object of thisinvention to provide a scale inhibitor of increased stability, allowingpremixing without premature precipitation of the scale inhibitor,particularly of phosphonate salts.

There was a need to find improved compositions suitable to inhibitscaling in a subterranean oil or gas producing formation. Suchcomposition should not contain solid components that may cause problemsduring the deployment. It is further desirable that the scale inhibitorcomposition may be delivered and stored as a single component in orderto reduce storage costs and deployment complexity. It has been foundthat adding to the formation a composition comprising metal chelant, ascale inhibitor and metal cations, wherein the stability constant of themetal chelant—metal cation chelate is higher than the stability constantof the chelate formed from the metal cations and the scale inhibitorwill inhibit scaling in a subterranean oil or gas producing formation.

DESCRIPTION OF THE INVENTION

The present invention provides a process for inhibiting scaling in asubterranean oil or gas producing formation, comprising adding to theformation a composition comprising a metal chelant, a scale inhibitorand divalent metal cations, wherein the stability constant of the metalchelant—metal cation chelate at ambient temperature is equal to orhigher than the stability constant of the chelate formed from the metalcations and the scale inhibitor, and wherein the solubility of thechelate formed from the metal cations and the scale inhibitor decreaseswith increasing temperature.

The present invention further provides a composition for inhibitingscaling in a subterranean oil or gas producing formation comprising ametal chelant, a scale inhibitor and divalent metal cations, wherein thestability constant of the metal chelant—metal cation chelate at ambienttemperature is equal to or higher than the stability constant of thechelate formed from the metal cations and the scale inhibitor, andwherein the solubility of the chelate formed from the metal cations andthe scale inhibitor decreases with increasing temperature.

The present invention further provides the use of a compositioncomprising a metal chelant, a scale inhibitor and divalent metalcations, wherein the stability constant of the metal chelant—metalcation chelate at ambient temperature is equal to or higher than thestability constant of the chelate formed from the metal cations and thescale inhibitor, and wherein the solubility of the chelate formed fromthe metal cations and the scale inhibitor decreases with increasingtemperature for inhibiting scaling in a subterranean oil or gasproducing formation.

The present invention relates to the inhibition of scale formation byutilising the precipitation reaction of a slightly insoluble scaleinhibitor salt within an oil producing formation. The slow release overtime of the scale inhibitor as it dissolves in the produced fluidsprotects the wellbore and production equipment from harmful scaledeposits.

The term ambient temperature refers to a temperature in the range of 0to 40° C.

The composition for inhibiting scale formation preferably comprisesmetal cations in a concentration of 1 to 10, specifically 2 to 8 wt.-%.

The composition for inhibiting scale formation preferably comprises atleast one scale inhibitor in a concentration of 10 to 50, specifically20 to 30 wt.-%.

The composition for inhibiting scale formation preferably comprises atleast one metal chelant in a concentration of 2 to 40, specifically 5 to15 wt.-%.

The composition for inhibiting scale formation may comprise water ad 100wt.-%.

The composition for inhibiting scale formation preferably comprises aphosphonate scale inhibitor.

The composition is preferably adjusted to a pH in the range of 4-14where, without the chelant, precipitation of the scale inhibitor wouldoccur immediately.

The composition of the invention is indefinitely stable at roomtemperature as a neat product, and when mixed in any proportion,preferably at 20-99 wt-% of the composition in a medium selected fromsea water and modified KCl brines such mixtures are also stableindefinitely. This is because unlike in the process disclosed in U.S.Pat. No .4,860,829 the metal chelant has a higher stability constantthan the phosphonate inhibitor. When the mixture in a medium selectedfrom sea water and modified KCl brines is placed downhole attemperatures of more than 40-150° C. this provides thermal conditionsfor the solubility limit of the metal ion-scale inhibitor chelate todecrease sufficiently to precipitate over a period of 1-24 hours,depending on pH adjustment. High pH levels will induce slowerprecipitation of the scale inhibitor, low pH levels will induce fasterprecipitation of the scale inhibitor. The metal ion-scale inhibitorchelate then precipitates in the formation allowing for slow release ofthe scale inhibitor in produced fluids. As the composition is stableuntil placed at a downhole temperature of more than 40-150° C. for 1-24hours no premature precipitation of the metal ion-scale inhibitorchelate is observed. This gives greater flexibility during pumping thanthe method disclosed in U.S. Pat. No. 4,860,829.

This technique for scale inhibitor precipitation is not limited to thechoice of phosphonate as scale inhibitor. The invention relies on thechelant having an equal or higher stability constant than the scaleinhibitor. This means that any scale inhibitor, preferably phosphonatesor a variety of polymer scale inhibitors may be deployed according tothis invention.

At low temperatures of about 50-70° C. no precipitation is observed forcompositions relying on the thermal break down of urea. The thermalbreakdown of urea is required to release the pH modifier according tothe process disclosed in U.S. Pat. No. 5,141,655. This is not requiredaccording to the current invention.

Careful modification of the composition and the pH allows for theprecipitation time to be modified for different downhole temperatures.Hence the current invention can be modified for application at highertemperatures (up to 150° C.) without the need for thermal decompositionto produce a pH modifier, such as the decomposition products of urea. Inorder to work the invention at higher temperatures of e.g. 120 to 150°C. the pH of the composition must be increased in order to achieveprecipitation of the scale inhibitor within a time of 2 to 24,preferably 4 to 18 hours. The exact extent of necessary pH increase isscale inhibitor and chelant specific. The extent of precipitation canalso be modified for specific needs. This can be done by varying theconcentration applied, or by varying the synthetic brine chemistry.

A variety of water soluble metal salts can be used to carry out thedescribed invention. These include as examples salts of metals such asiron, manganese, calcium, cadmium, zinc, tin, nickel, magnesium andbarium. Calcium and magnesium are the preferred salts, in particularcalcium. In a further preferred embodiment calcium and magnesium areused. The specific salts to be employed are preferably the chlorides ornitrates of the above metals. This invention, however, can be carriedout with any metal salt forming a complex with a chelating agent and atdeployment conditions the chelating agent releases the metal cationswhich then form the partially insoluble metal ion-scale inhibitorchelate.

The metal chelate must have a stability constant equal to or higher thanthat of the scale inhibitor. Thus the preferred chelating agents(chelants) are 1,2-diaminocyclohexane-N,N′-tetraacetic acid,1,2-bis(2(dicarboxymethyl)aminoethoxy)ethane, ethylene diamine tetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA) ,aminobarbituric acid-N,N-diacetic acid, nitroacetic acid, salicylicacid, b-hydroxy butyric acid, 4-sulfoaniline diacetic acid, lactic acid,glycolic acid, glyceric acid, gluconic acid, a-alanine, 3-sulfoanilinediacetic acid, 4-aminobenzoic acid-N,N-diacetic acid, adenosinephosphate, glycine, 3-aminobenzoic acid-N,N-diacetic acid, serine,tyrosine, aniline diacetic acid, N-butylethylenediamine-triacetic acid,aspartic acid, glutamic acid, N-cyclohexylethylenediamine-triaceticacid, N,N′-ethylenebis(2(o-hydroxyphenyl))glycine, tartaric acid, malicacid, b-(N-trimethylammonium ethylimino diacetic acid, disodium,2-dihydroxybenzene-3,5-sulfonate, imino-diacetic acid,N-Cyanomethylimino-diacetic acid, adenosine di-phosphate,N-carbethoxy-b-aminoethylimino-diacetic acid, tri-polyphosphoric acid,citric acid,

N-methylthioethylimino-diacetic acid, tri-metaphosphoric acid,8-hydroxyquinoline-5-sulfonic acid, adenosine tri-phosphate,N-methyl-imino-diacetic acid, N-acetamidoimino-diacetic acid,b-aminoethylsulfonic acid-N,N′-tetracetic acid,N-methoxyethylimino-diacetic acid, 2-sulfoaniline diacetic acid,pentamethylenediamine-tetraacedic acid, N-hydroxyethylimino-diaceticacid, ethylenediamine-N,N-diacetic acid,1,3-diaminocyclohexane-N,N′-tetraacetic acid,b-mercaptoethylimino-diacetic acid, tetra-metaphosphoric acid, nitrilopropionic diacetic acid, tetramethylenediamine-tetraacetic acid,2-aminobenzoic acid-N,N-diacetic acid, b-aminoethylphosphonicacid-N,N-diacetic acid, N,N-Dihydroxyethylethylenediamine-diacetic acid,ethylendiamine-tetra(methylenephosphonic) acid, nitrilo triacetic acid,N-benzylethylenediamine-triacetic acid, trimethylenediamine-tetraaceticacid, aminomethylphosphonic acid-N,N-diacetic acid, orN-hydroxyethylenediamine-triacetic acid.

Suitable scale inhibitors include diethylenetriamine penta(methylenephosphonic acid), or nitrilo(methylene phosphonic acid) although anyphosphonate scale inhibitor can be used as well as a number of polymerbased scale inhibitors. These can include methacrylic diphosphonatehomopolymer, acrylic acid-allyl ethanolamine diphosphonate copolymer,SVS (sodium vinyl sulphate)-acrylic acid-allyl ammonia diphosphonateterpolymer, acrylic acid-maleic acid-DETA (diethylene triamine) allylphosphonate terpolymer, polyaspartic acid, polycarboxylates.

Preferred substances for pH adjustment include KOH, NaOH, NH₃ and anyamine containing compound.

For the purpose of this invention, the stability constant is defined as

$K = \frac{\lbrack{ML}\rbrack}{\lbrack M\rbrack \lbrack L\rbrack}$

whereinK means the stability constant,[ML] means the concentration of the chelate formed from metal cationsand either the metal chelant or the scale inhibitor,[M] means the concentration of the metal cations, and[L] means the concentration of either the metal chelant or the scaleinhibitor.

EXAMPLES

All percentages within this disclosure are weight percentages withrespect to the total weight of the respective composition, unlessotherwise noted.

Example 1

A composition containing 3 wt.-% CaCl₂.2 H₂O, 6 wt.-% diethylenetriaminepentaacetic acid (DTPA) and 30 wt.-% diethylenetriamine penta(methylenephosphonic acid) (DETPMP), the pH adjusted with NaOH to between 4.4 and6.2 was placed in a sample of synthetic sea water. The stabilityconstants for both the DTPA and DETPMP with calcium is 10¹⁰. Thesolution was completely stable at room temperature for a period of 60days.

The solution was placed in an oven at 52° C. and monitored for changesin turbidity over time. The time to precipitation was adjusted bychanging the neat composition pH. Precipitates were visible within atime period of 2 to 18 hours:

pH Time to precipitation (hrs) 5.8 0.8 6.1 1.25 6.2 2.25 6.3 10 6.4 22.56.7 no precipitation

When removed from the temperature of 52° C. and returned to roomtemperature the precipitate dissolved over a couple of weeks. Thisindicates that unlike to the process disclosed in U.S. Pat. No.4,860,829 the chelant does not decompose and that the stability constantchanges at elevated temperatures releasing the metal cations. Thus thecalcium chelate of diethylenetriamine penta(methylene phosphonic acid)precipitates. It is believed that when returned to ambient conditionsthe stability constant of the DTPA returns to its higher value andcalcium ions are removed from the phosphonate.

Example 2

A composition containing 3 wt.-% CaCl₂.2 H₂O, 5 wt.-% dithylenetriaminepentaacetic acid (DTPA) and 25 wt.-% Dequest® 2086 (hexamethylenephosphonate) was pH adjusted with KOH to between 7 and 10. A 20 wt.-%solution of the described composition was prepared in a modified KClbrine containing dissolved metal halide salts. The solution was placedin an oven at 52° C. and monitored for changes in turbidity over time.The time to precipitation was adjusted by changing the neat compositionpH

pH Time to precipitation (hrs) 7.2 1:10 8.6 (and higher) noprecipitation

Example 3

A composition containing 3 wt.-% CaCl₂.2 H₂O, 6 wt.-% ethylene diaminetetra acetic acid (EDTA) and 30 wt.-% diethylenetriamine penta(methylenephosphonic acid), pH adjusted with NaOH to 6.9 was placed in a sample ofsynthetic sea water. The solution was placed in an oven at 114° C. andprecipitated within a time period of 1.5 hours. This shows that with pHmodification the current invention can be utilised at a wide range oftemperatures.

Example 4

A composition consisting of 7 wt-% CaCl₂. 2 H₂O, 28% diethylenetriaminepentaacetic acid (DTPA) and 27% of a biodegradable polymaleic acid scaleinhibitor was prepared at a pH of 12.5-13. The solution was placed in anoven at 132° C. and it precipitated after 3 and a half hours.

Example 5

A composition consisting of 7 wt-% CaCl₂.2 H₂O, 28% of trisodium salt ofmethylglycine diacetic acid (Trilon® M BASF >80% biodegradable) and 27%of a biodegradable sulphonated co-polymer scale inhibitor was preparedat a pH of 14. As there is an ever growing need for bio-degradable scaleinhibitors and chelants this package provides both. The solution wasplaced in an oven at 132° C. and precipitated after an hour and 15minutes.

1-8. (canceled)
 9. A composition comprising a metal chelant, a scaleinhibitor and 1 to 10 wt.-% of divalent metal cations, wherein thestability constant of the metal chelant—metal cation chelate at ambienttemperature is equal to or higher than the stability constant of thechelate formed from the metal cations and the scale inhibitor, andwherein the solubility of the chelate formed from the metal cations andthe scale inhibitor decreases with increasing temperature, thecomposition having a pH of 4 to
 14. 10. A process for inhibiting scalingin a subterranean oil or gas producing formation comprising the step ofadding the composition according to claim 9 to the subterranean oil orgas producing formation.